Line-type inkjet recording devices well known in the art are equipped with a line head having nozzles arranged over the maximum printing width of a printing medium. In this line-type inkjet recording device, the line head can remain fixed while the printing medium is conveyed a prescribed distance after the line head prints each line. Hence, this line-type inkjet recording device has the advantage of being able to print faster than serial-type inkjet printing devices that print while reciprocating a print head.
Next, a description will be given with reference to FIG. 1 on the arrangement of nozzles in this conventional line-type inkjet recording device and the relationship of these nozzles to the dots formed on a printing medium by ink droplets ejected from the nozzles. FIG. 1(a) illustrates the ideal relationship between the nozzles and dots formed on the printing medium for the conventional line-type inkjet recording device.
As shown in FIG. 1(a), a line head 101 has five nozzles 102a-102e arranged linearly (in the left-to-right direction in FIG. 1) at a prescribed pitch P. Here, the nozzles 102a-102e are actually not arranged along the same line but are each disposed in one of a plurality of lines in the line head. In other words, FIG. 1 shows the nozzles 102a-102e that eject ink droplets for forming dots within the same line as being themselves arranged in the same line.
Further, the nozzles 102a-102e eject ink droplets toward a printing medium (not shown) as the printing medium is conveyed in a conveying direction H (downward in FIG. 1) to a position opposite the line head 101. The ink droplets ejected from the nozzles 102a-102e impact the printing medium and form dots of a size sufficient to circumscribe square pixels (indicated by dotted lines in FIG. 1(a)).
Hence, when these five nozzles 102a-102e eject ink droplets vertically toward the printing medium, the ink droplets impact the printing medium to form five overlapping dots arranged in a straight line (the left-to-right direction in FIG. 1) at a prescribed pitch D.
By repeating the operation described above at a prescribed timing while conveying the printing medium, ink droplets ejected from the nozzle 102a form a column of dots A arranged vertically in FIG. 1. Similarly, the nozzles 102b, 102c, 102d, and 102e produce columns of dots B, C, D, and E having no gaps therebetween.
Although the ejected ink droplets are expected to follow a vertical trajectory toward the printing medium, ink droplets are sometimes ejected along a slanted trajectory relative to the printing medium due to various reasons, such as dust, solidified ink globules, and the like obstructing one of the nozzles 102a-102e or ink deposited around the periphery of the nozzle pulling against the ejected ink droplet.
FIG. 1(b) shows the relationship between the nozzles and dots formed by ink droplets ejected from the nozzles when ink droplets ejected from the nozzle 102c follow a slanted trajectory relative to the printing medium due to one of the reasons described above.
As shown in FIG. 1(b) when the nozzle 102c ejects ink droplets at a slant to the printing medium (toward the nozzle 102d), the ink droplets form a column of dots C having a bias toward the column of dots D so that a pitch D2 between the columns of dots B and C is greater than the prescribed pitch. Consequently, a gap is produced between the columns of dots B and C that appears as a streak along the conveying direction H of the printing medium, lowering the quality of the image.
To resolve this problem, Patent Reference 1 given below discloses an inkjet printer comprising means for vibrating the line head 101 described above. By vibrating the line head 101 with the head vibrating means of this technology, ink droplets ejected from the nozzles also vibrate in response, decreasing the gap described above and thereby preventing a drop in image quality.
Another technique for resolving the problem described above is disclosed in Patent Reference 2 given below. In this technology, a plurality of heaters capable of being driven independently of one another is provided for a single nozzle, the heaters being provided at different positions in an ink chamber corresponding to the nozzle. This technology changes the heater being driven and the driving force of the heater for each line. Hence, this technology can vary the positions at which the ink droplets impact the printing medium, thereby reducing the gap described above and preventing a drop in image quality.
Patent Reference 1: Japanese unexamined patent application publication No. HEI-10-235854 (paragraph 18, FIG. 2, etc.)
Patent Reference 2: Japanese unexamined patent application publication No. 2002-240287 (paragraph 52, etc.)